James Hansen is the director of the NASA Goddard Institute for Space Studies. The following essay was excerpted from a presentation given Dec. 6 at the annual meeting of the American Geophysical Union.

San Francisco — The Earth’s temperature, with rapid global warming over the past 30 years, is now passing through the peak level of the Holocene, a period of relatively stable climate that has existed for more than 10,000 years. Further warming of more than 1 degree Celsius will make the Earth warmer than it has been in a million years.

“Business-as-usual” scenarios, with fossil fuel CO2 emissions continuing to increase at 2 percent per year as in the past decade, will yield additional warming of 2 degrees Celsius or 3 degrees Celsius this century. Such a drastic increase would imply changes that constitute practically a different planet.

The Earth’s climate is nearing, but has not passed, a tipping point, beyond which it will be impossible to avoid climate change with far-ranging undesirable consequences. The changes include not only loss of the Arctic as we know it, with all that implies for wildlife and indigenous peoples, but losses on a much vaster scale due to worldwide rising seas.

Sea level will increase slowly at first, as losses at the fringes of Greenland and Antarctica due to accelerating ice streams are nearly balanced by increased snowfall and ice sheet thickening in the ice sheet interiors. But as Greenland and West Antarctic ice is softened and lubricated by melt-water, and as buttressing ice shelves disappear due to a warming ocean, the balance will tip toward ice loss, thus causing rapid ice sheet disintegration.

The Earth’s history suggests that with warming of 2 degrees Celsius to 3 degrees Celsius, the new equilibrium sea level will include not only most of the ice from Greenland and West Antarctica, but a portion of East Antarctica, raising sea level 25 meters (80 feet).

Within a century, coastal dwellers will be faced with irregular flooding associated with storms. They will have to continually rebuild above a transient water level.

This grim “business-as usual” scenario of climate change can be halted if growth of greenhouse gas emissions is slowed in the first quarter of this century. The goal of keeping further global warming under 1 degree Celsius to avoid crossing the tipping point requires two things: first, flattening out and then decreasing the rate of growth of CO2 emissions, primarily through improvement in energy efficiency, and, second, an absolute decrease in emissions of non-CO2 gases that also affect warming, particularly methane and carbon monoxide, and therefore tropospheric ozone, and black carbon (soot) aerosols.

Action must be prompt. Otherwise CO2-producing infrastructure that may be built within a decade will make it impractical to keep further global warming under 1 degree Celsius. Of top concern is the large number of coal-fired power plants that China, the United States and India are planning to build without CO2 sequestration (a process whereby CO2 is separated from the energy produced and stored in the ground).

The CO2 Problem | The great interest in CO2 is due to the realization that increasing CO2, other things being equal, will cause global warming. CO2 is a greenhouse gas. It absorbs the Earth’s infrared radiation, reducing the emission of heat to space. This causes a temporary imbalance between the amount of solar energy absorbed by the Earth and the energy emitted to space. So the Earth will warm up until it restores energy balance.

Global warming in just the past 30 years is more than one-half degree Celsius, about 1 degree Fahrenheit in 30 years.

The good news about CO2 is that about 40 percent of annual fossil fuel emissions continue to be soaked up. And if we decreased CO2 emissions and improved reforestation and agricultural practices, we could probably increase the percentage uptake. The bad news is that stabilization of atmospheric CO2 amount may require reducing emissions by as much as 60 percent to 80 percent. But, on the contrary, emissions are still increasing—2 percent per year in the past decade.

Is continued growth of this sort inevitable, or is there a feasible alternative course? In the long run, satisfying energy needs while decreasing CO2 emissions will require development of renewable energies, sequestration of CO2 produced at power plants, and perhaps a new generation of nuclear power. But a flattening out of emissions can be achieved now with improved energy efficiency. It is important that the United States, as a technology leader and as the largest producer of CO2 in the world, take a leadership role.

In general, industrial emissions of CO2 are declining. The two problems are emissions from power plants and emissions from vehicles. The solution in both cases depends critically on efficiency. With power plants, we need to avoid building a fossil fuel power plant infrastructure unless and until sequestration is a reality. For vehicles, efficiency is critical because of the rapidly growing global number of vehicles. It is false to say that hydrogen technology will solve this in the future. It takes energy to make hydrogen. Efficiency will always be needed. By achieving it now, we could readily get onto an alternative scenario track.

In the US, even though the number of vehicles on the road increases every year, we could get off the path of increasing emissions by accepting even the moderate recommendations that call for phasing in improvements in efficiency that would amount to about 30 percent by 2030. This would be based on available technology, and it gives the automakers ample time to phase it in.

The accrued benefit in 35 years, of just this moderate action, even without addition of hydrogen-powered vehicles, is a savings of oil equal to more than seven times the amount of oil that the US Geological Survey estimates to be available in the Alaska National Wildlife Refuge.

As a technical matter, halting the rise of global temperature to below 1 degree Celsius is totally within reach. Everything now depends on an informed public to bolster the political will of leaders across this warming globe.